Method of coating paper



w. w. YHERRICK METHOD OF COATING PAPER Filed April 5, 1942 ATTORNEYS MQQZ A v l(|tIl\lll |||ll|||ll uulllllllllll l v in I J I I v I Maya,1945'.

Patentegl May 8, 1945 METHOD OF COATING PAPER Walter w. Herrick,Milford, N. J., assignor to Riegel Paper Corporation, New York, N. Y., acorporation of New Jersey I Application April 3, 1942, Serial N 0.437,573

6 Claims.

This invention relates to a method of coating paper, and moreparticularly to a method of applying a uniform coating of a materialcapable of being liquefied by heat to both sides of a paper sheet orweb.

Various coatings are applied to paper which are capable of beingliquefied at temperatures which can be safely employed in the treatmentof paper and which are then fluid enough to form uniformly levelcoatings. The coatings, after hardening, are capable of being remeltedon a packaging machine to form a self-sealing covering. One example ofsuch coating material is wax. The application of wax coatings of lowmelting pointto paper presents no particular difficulties. At thetemperatures employed in applying the coating the wax is suflicientlyfluid to flow on the sheet and form a uniformly level coating.

The use of higher melting point waxes and other coatings,' however, donot produce uniform coatings by the methods heretofore employed andrequire further treatment. The higher melting point waxes and othermaterials are not sufficiently fluid to flow on the sheet and form auniformly level coating, at temperatures which can be safely employed intreating paper, and it has therefore been necessary to smooth or levelthe coating by means of doctor blades or other similar mechanism. I havefound that uniform coatings of materials, which are too viscous to flowat coating temperatures, can be obtained by passing the coated paperthrough a molten bath of a low melting alloy or metal.

I am aware that coatings of thermo-setting materials have been appliedto objects to be coated and then passed through a bath of mol- .coatedpaper through a molten bath of a low melting point metal or alloywithout affecting the moisture content of the coated paper.

The process may be practiced in various types the molten bath. A simplemethod of practic ing the process is to place a body of wax or othercoating material on the top of the bath at the inlet side of the machinewhereby the coating tity of a low melting metal or alloy 2.

is applied to the paper as it passes through this layer of wax or othercoating material and is then smoothed by passing through the moltenmetal. I

In the accompanying drawing I have diagrammatically illustrated one typeof apparatus for practicing the process in this manner. In the drawing:

Fig. 1 is a vertical, sectional view of a tank or container; and

Fig, 2 is a plan View.

Referring to the drawing, the reference numeral l designates generallythe tank or container. While it is illustrated as a V-shaped tank invertical cross section, any other type of tank in which the paper sheetor web can be carried through the molten bath may be employed. Withinthe tank I place a suitable quan- Suitable heating means 3 may bedisposed at various points in the tank to maintain the metal in moltencondition. In the drawing I have illustrated a body of high meltingpoint wax 4 on the metal at the inlet side of the machine. The top of.the metal at this side of the machine is indicated by the dotted line 5,and the top of the metal at the outlet side of the machine is indicatedby the dotted line 6. Due to the greater specific gravity of the metal,the wax or other coating material is maintained in the position shownand the heat employed in maintaining the metal in a molten conditionkeeps the wax .or other coating material liquefied. To prevent the waxfrom flowing to the-outlet side of the machine, I provide a partition 1extending downwardly from the top of the tank to a roll or smoothcylindrical rod 8. The member 8 may be r a smooth stationary rod or maybe a rotating roll. The web, indicated by W passes into the machine atthe inlet side and is carried downwardly through the liquefied body 4 ofhigh melting point wax or other material. A suitable quantity of waxadheres to the surface of the paper and the paper then passes downwardlyover the rod 8 and upwardly to the outlet side of the machine. Inpassing through the molten metal, the wax or other coating material isconverted into a smooth coating of substantially uniform thicknss. Insome instances I may employ doctor blades 9 secured to the side walls ofthe tank and engaging the opposite sides of the paper to remove excesscoating material from the surfaces of the web. I In practicing theprocess I preferably employ a metal or alloy having a melting pointbelow 600 F. In a specific instance I have used an alloy consisting of54 parts bismuth, 26 parts tin and 20 parts cadmium which has a. meltingpoint .of 220. I may, however, employ alloys or metals of higher meltingpoints and may employ either tin or lead or other metals or alloyshaving melting points comparable to the melting points of tin and lead.

As stated, the paper may have a suitable quan-, tity of wax or othercoating material applied to it before entering the tank I and the layer4 of liquefied wax or other coating material may be eliminated. I find,however, that the simplest way of practicing the process is that hereinillustrated and described. The thickness of the coating may becontrolled by adjusting the doctor blades 9. The thickness of thecoating may also be regulated by controlling the speed at which thepaper is fed through the tank or by varying the depth of the layer 4 ofcoating material. In describing the process, paper has been used by wayof example. The process, however, may be, applied to other fibrous websand to webs of regenerated products, films and the like. By passing suchflammable materials through the heated bath of molten metal after thecoating material has been applied to the surface, the web is protectedfrom excess heat and the moisture content is maintained withoutalteration. Any suitable heating means may be employed in the heating} Aheating fluid, such as oil, steam, or water, may be employed, orelectrical heating,

elements 3.

units may be placed within the pipes or tubes 3. Other low melting pointalloys which may be employed are Rose metal consisting of substantially27.5 percent lead, 48.9 percent bismuth and 23.6 percent tin; and Woodsmetal consisting of substantially 25.85 percent lead, 6.99 percentcadmium, 52.43 percent bismuth and 14.73 percent tin.

As'stated above, the process may be advantageously applied to coatingmaterials which, when melted or softened by heat at a temperature whichthe paper will withstand, are of such vis cosity that it will notfiow'to form substantially uniformly level coatings.

Such materials are generallydefined in the paper industry as hot meltsExamples of such materials are high 1 melting point waxes, ethylcellulose, ester gum,

rosin, Pliolite and hydrogenated oils. These materials may be used alonebut are frequently 1 employed in admixture with low melting point waxesand in admixture with each other and other materials. Such mixturesfrequently do not have a true melting point, and the invention thres ofmaterials, some of which may have rela tively low melting points, butwhich are too viscous at such temperatures to form uniformly levelcoatings.

In the claims the expression high melting point waxes is intended toinclude mixtures of wax and other materials which are not sumcientlyfluid to form a substantially uniformly level coating at temperaturesusually employed in coating paper. These temperatures are usually in theneighborhood of 190 to 200 F., although with some types of paper, highertern 5 peratures may be employed.

I claim:

1. The herein described method which comprises maintaining a molten bathof bismuth, tin

i is applicable either to coatings which, when i melted at temperatureswhich can be employed andcadmium in a container, maintaining a layer" ofhigh melting point wax'on the wear the bath at the inlet end of thecontainer, and' passing a web through the layer of wax and the moltenbath whereby a coating of wax is first applied to the web and thecoating then levelled as the web passes through the molten bath.

2. The herein described method which comprises maintaining a molten bathof a metal having a melting point below 600 F. in a container,maintaining a layer of high melting point wax on the top of the bath atthe inlet end of the container, and passing a web through the layer ofwax and the molten bath whereby a coating of wax is first applied tothe'web and the coating then leveled as the web passes through themolten bath, the melting point of the wax being lower than the meltingpoint of the metal.

3. The method of forming substantially uniform level coatings of highmelting coating material on fibrous webs in a continuous manner whichincludes continuously applying to the web a high melting coatingmaterial in a viscous state at a temperature at which the coatingmaterial will not flow to form substantially uniform level coatings andthen passing the coated web through a hot molten metal bath to convertthe coating material into a smooth coating of substantially uniformthickness.

4. The method of coating paper with a high melting coating material in acontinuous manner which includes continuously applying to both sides ofthe paper a high melting coating material in a viscous state at atemperature at which it will not flow to form substantiallylevelcoatings on the paper and continuously passing the thus coatedpaper through a hot bath of molten metal with resulting subjection ofthe coated web to compression by the molten metal and brushing of thecoated sheet in its passage through the molten metal to smooth thecoatings and form substantially uniform level coatings.

5. The method of coating fibrous webs with a high melting point coatingin a continuous manner which includes maintaininga molten layer of ahigh melting coating material in a viscous condition at a temperature atwhich the material will not flow to form substantially uniform levelcoatings superimposed on top of a molten metal bath maintained at atemperature about the melting point of the coating material,continuously passing the fibrous web downwardly through the layer ofcoating material and thereby coating both sides of the web with theviscous coating material, continuously passing the coated web downwardlyinto and through v the metal bath, whereby the coated web is subjectedto compression by the molten metal and to a brushing action thereby andthe coating layers are converted into substantially uniform levelcoatings of the high melting point material, and subsequently coolingthe resulting coated 'web.

6. The method of coating paper in a continuous manner with a highmelting wax which includes applying to both sides of the paper in acontinuous manner a high melting point wax in a viscous liquid conditionat a temperature at which the wax does not form substantially uniformlevel coatings and continuously passing the thus coated paper into andthrough a molten metal bath at a temperature above the melting point ofthe wax to level the wax and form substantially uniform level coatingsthereof on the paper and subsequently cooling the coated .paper.

WALTER W. HERRICK.

